This invention relates to a method of forming a barrier layer in a high aspect ratio recess in a dielectric layer on a semiconductor wafer.
It is well known to form barrier layers on semiconductor wafers to prevent the metalisation material, e.g. aluminium alloy or copper, spiking into the underlying dielectric layer or to prevent WF6 (used in the deposition of tungsten) reacting with a titanium layer. Commonly these comprise a Ti seed layer and an overlying layer of stoichiometric TiN. Many methods have been proposed for forming such layers but they nearly all run into considerable difficulties when trying to form a satisfactory barrier layer deep in a high aspect ratio recess within the dielectric layer. As the features become smaller and smaller in semiconductor wafers, the aspect ratio of such recesses increases. Normally the sputtering distances for Ti and TiN are around 45 mm and sputtering typically takes place at a pressure around 5 millitorr. Due to problems of directionality, particularly when nitrogen is present to form TiN, the barrier layers formed in high aspect ratio holes are often not very successful.